A prior method of preheating a thermoformable material, including thermoset and thermoplastic materials, involves contact heating. The contact heating method includes placing the material in direct contact with a heat source, such as thermally regulated upper and lower platens. Heat is then transferred principally by conduction from the outer surface to the inner core of the material. Because such materials are typically poor conductors of heat, however, this method requires a significant amount of time to sufficiently heat the materials.
Another method of preheating a thermoformable material involves radiant heating. This method involves placing the material near a radiant heat source, such as electric coils or ceramic heaters. The outer portions of the material, however, tend to selectively absorb the radiant energy, and core heating is again accomplished primarily by conduction. Consequently, this method also requires a significant amount of time to sufficiently heat the material.